The present invention relates to electrical insulating paper used in the oil-impregnated state.
To meet the ever increasing demand for electrical power in recent years, it has been desired to develop techniques for transmitting large blocks of power through extra-and ultra-highvoltage oil-filled underground cables. The development of high-quality electrical insulating paper is one of the important keys to the establishment of such technique. Because the kraft pulp insulating paper is not suited for use in extra-high-voltage cables due to its high dielectric loss factor, research efforts have been directed throughout the world to the development of electrical insulating paper having lower loss than kraft pulp insulating paper. In fact, many proposals are so far known.
Polypropylene, with its low loss factor and high thermal stability, is one of the most hopeful polymers as a substitute for kraft pulp in making insulating paper. Electrical insulating papers consisting predominantly of polypropylene are known in U.S. Pat. nos. 3,532,800, 3,194,872, 3,749,813, 3,957,573, etc. However, the papers disclosed in these patents are not fully satisfactory in oil resistance. The paper proposed in U.S. Pat. No. 3,957,573, although having much higher oil resistance than the others, still involves the problem that the elastic modulus of the paper gradually decreases during a prolonged period of use in the oil-impregnated state. Japanese Patent Publication No. 8421/1963 dicloses mixed paper composed of kraft pulp and polypropylene fibers. The proposal has the drawback that an attempt to lower the dielectric loss factor of the paper invariably reduces its mechanical strength due to the low bonding strength between the kraft pulp and polypropylene fibers.